Turbofan aircraft engines have large fans at the forward end. They rotate at a high speed at about 3000 revolutions per minute.
Foreign objects such as birds, hailstones or debris ingested from the ground occasionally strike the fan blades. It is possible for this to cause the fan blades to fragment. These fragments may be on the order of 7 kilograms traveling at about 930 meters per second. It is essential to contain the blade fragments and also to retain the casing.
A typical containment structure is shown in U.S. Pat. No. 4,490,092 entitled "Containment Structure" and issued to Emile J. Premont. A support structure has "c" shaped stiffeners between inner and outer sheets. This structure surrounds the fan and has multiple layers of woven KEVLAR.RTM. ballistic fabric (trademark of DuPont Corporation). This fabric is wound under tension and serves to resiliently contain blade fragments passing through the support structure.
It is critical to contain fragmented blades without disintegration or collapse of the casing against the blade. The casing is more susceptible to collapse aft of the penetration zone where fabric restraint forces on the case and rebound forces peak. It is important that the containment structure be lightweight.
Isogrid structures are known and used in space vehicles and aircraft because of the high stiffness with low weight. The structures consist of a triangulation of stiffening members with or without a skin on one side. Isogrid structures had been used generally as a portion of containment structures for turbofans.